The general process for the manufacture of biomolecules, such as proteins, particularly recombinant proteins, typically involves two main steps: (1) the expression of the protein in a host cell, followed by (2) the purification of the protein. The first step involves growing the desired host cell in a bioreactor to effect the expression of the protein. Some examples of cell lines used for this purpose include Chinese hamster ovary (CHO) cells, myeloma (NSO) bacterial cells such as e-coli and insect cells. Once the protein is expressed at the desired levels, the protein is removed from the host cell and harvested. Suspended particulates, such as cells, cell fragments, lipids and other insoluble matter are typically removed from the protein-containing fluid by filtration or centrifugation, resulting in a clarified fluid containing the protein of interest in solution as well as other soluble impurities.
The second step involves the purification of the harvested protein to remove impurities which are inherent to the process. Examples of impurities include host cell proteins (HCP, proteins other than the desired or targeted protein), nucleic acids, endotoxins, viruses, protein variants and protein aggregates. This purification typically involves several chromatography steps, which can include affinity chromatography, ion exchange, hydrophobic interaction, etc. on solid matrices such as porous agarose, polymeric or glass or by membrane based adsorbers.
One example of a chromatography process train for the purification of proteins involves protein-A affinity, followed by cation exchange, followed by anion exchange. The protein-A column captures the protein of interest or target protein by an affinity mechanism while the bulk of the impurities pass through the column to be discarded. The protein then is recovered by elution from the column. Since most of the proteins of interest have isoelectric points (PI) in the basic range (8-9) and therefore being positively charged under normal processing conditions (pH below the PI of the protein), they are bound to the cation exchange resin in the second column. Other positively charged impurities are also bound to this resin. The protein of interest is then recovered by elution from this column under conditions (pH, salt concentration) in which the protein elutes while the impurities remain bound to the resin. The anion exchange column is typically operated in a flow through mode, such that any negatively charged impurities are bound to the resin while the positively charged protein of interest is recovered in the flow through stream. This process results in a highly purified and concentrated protein solution.
Other alternative methods for purifying proteins have been investigated in recent years. One such method involves a flocculation technique. In this technique, a soluble polyelectrolyte is added to an unclarified cell culture broth to capture the suspended particulates and a portion of the soluble impurities thereby forming a flocculant, which is subsequently removed from the protein solution by filtration or centrifugation.
Alternatively, a soluble polyelectrolyte is added to clarified cell culture broth to capture the biomolecules of interest, thereby forming a flocculant, which is allowed to settle and can be subsequently isolated from the rest of the solution. The flocculant is typically washed to remove loosely adhering impurities. Afterwards, an increase in the solution's ionic strength brings about the dissociation of the target protein from the polyelectrolyte, subsequently resulting in the resolubilization of the polyelectrolyte into the protein-containing solution.
In co-pending application U.S. Ser. No. 12/004,314 filed Dec. 20, 2007, the disclosure of which is hereby incorporated by reference, a polymer, soluble under certain conditions, such as temperature, pH, salt, light or combinations thereof, is used to bind impurities while in its soluble state and is then precipitated out upon a change in condition (pH or temperature, etc.) removing the impurities with it. The biomolecule of interest is then further treated using traditional chromatography or membrane adsorbers and the like.
All of the protein purification technologies discussed above share a common theme, namely, to first remove suspended particulates in a first distinct step and then in a second step separate the biomolecules of interest from soluble impurities which are inherent to the process.
In situ product recovery with derivatized magnetic particles is one example of a protein purification technique where the biomolecules of interest can be purified directly from an un-clarified cell culture broth. In this technique, a polymer shell encapsulating a magnetic bead is functionalized with an affinity ligand that seeks out and binds the target protein. A magnetic field is then applied to collect the bead-protein complexes, leaving behind the soluble impurities and insoluble particulates.
The main drawback of this technique is that it requires appreciable capital investments in design, construction and validation of high-gradient magnetic separators. Also, the technique does not lend itself to disposable applications, which are poised to become the norm for protein purification in the Bioprocess industry.
In co-pending application filed Dec. 16, 2008 under Ser. No. 12/316,708, entitled “Purification of Proteins” by Moya, Wilson, et al., the disclosure of which is hereby incorporated by reference, there is disclosed a polymer such as a soluble polymer capable of substantially irreversibly binding to insoluble particulates and a subset of soluble impurities and also capable of reversibly binding to one or more desired biomolecules in an unclarified biological material containing stream and the methods of using such a material to purify one or more desired biomolecules from such a stream without the need for prior clarification. More specifically, this co-pending application discloses a stimuli responsive polymer such as a selectively soluble polymer capable of selectively and reversibly binding to one or more desired biomolecules in an unclarified biological material containing stream and the methods of using such a polymer to purify one or more desired biomolecules from such a complex mixture of materials including the biomolecule(s) of interest and various impurities such as other proteins (host cell proteins), DNA, virus, whole cells, cellular debris and the like without the need for prior clarification of the stream.
The polymer is soluble under a certain set of process conditions such as one or more of pH, salt concentration, temperature, light, or electrical field, and is able to interact and complex with insoluble impurities (cells, debris, etc.) and a fraction of the soluble impurities, and is rendered insoluble and precipitates out of solution upon a change in conditions (temperature, salt concentration, light, electrical field, or pH), e.g. a stimuli responsive polymer. Only when precipitated out of solution, the polymer is capable of reversibly binding to one or more desired biomolecules within the stream (protein, polypeptide, etc.) in an unclarified cell broth. The precipitate can then be removed from the stream, such as by being filtered out from the remainder of the stream and the desired biomolecule is recovered such as by selective elution from the precipitate.
The removal of the precipitate, however, can be problematic, as it is typically in the form a large mass of sludge.
It would be desirable to provide an apparatus and method for the efficient purification of samples, particularly those containing biomolecules, preferably within a single, integral, apparatus that reduces or eliminates one or more process steps that can result in contamination or material loss.